The present invention relates to microstrip antennas for use in the transmission and receipt of microwave signals. More specifically, the present invention relates to microstrip antenna structures which are configured in a curved manner so as to be conformally attached to the curved outer surface of an airframe.
Microstrip antennas are currently constructed using a single layer of metal clad plastic laminate. The laminate typically takes on a planar structure having electrically conductive metallic films on opposite surfaces thereof. Antenna radiators, matching networks, phase shifters and power dividers are formed by photochemically etching one side of the laminate. On the second side of the laminate remains a continuous metallic ground plane. This laminate is then curved or reconfigured to allow conformal attachment to the outer skin of an aircraft. A second sheet of protective plastic is then bonded to the antenna radiating surface, forming a protective radome.
The microstrip antenna structure comprises a single plastic laminate between the etched circuits and the ground plane. The thickness of this laminate is a function of the antenna frequency and bandwidth requirements.
As previously mentioned, it is necessary to curve or reconfigure the antenna structure so as to allow it to be conformally attached to the exterior surface of an airframe. Often, this requires curving the antenna structure into a very tight radius of curvature (e.g., forming a curved member having a radius of curvature as low as 2.5 inches). Typically, the forming or curving of the antenna structure is done by heating the microstrip antenna structure and pressing it into a mold.
As the laminate thickness increases, it becomes impossible to construct an antenna that will conform to a tight radius of curvature. Attempting to curve a microstrip antenna structure having a thick laminate into a very tight radius of curvature results in wrinkling or kinking of the ground plane. This wrinkling or kinking causes cracks and stresses in the ground plane, thus resulting in electrically intermittent portions. Furthermore, the wrinkling is objectionable for cosmetic reasons as it causes the part to look defective.
The kinking or wrinkling of the ground plane often necessitates scrapping or reworking the antenna. Rework or scrapping severely affects the efficiency of producing antennas, thus drastically affecting the cost of producing a conformal antenna.
Another problem with the antenna structure just described is the relaxing or straightening that results over time. The antenna is initially bent or curved to its desired radius of curvature. However, over time the antenna structure relaxes to a larger radius of curvature. This relaxation of the antenna configuration requires reworking and reforming the antenna, resulting in higher costs and higher scrap rates.